1. Field of the Invention
The present invention generally relates to laminated printed circuit boards, and more particularly to an embedded capacitor structure for laminated printed circuit boards.
2. The Prior Arts
Embedding capacitors in laminated printed circuit boards (PCBs) were originally disclosed by a U.S. company, Zycon, in 1992. In a PCB's laminated process, Zycon proposed to add extremely thin (e.g., 2-4 mil) dielectric layer and copper foil adjacent to the power plane or ground plane of the PCB so as to form embedded capacitors integral to the PCB. At that time, it was referred to as buried capacitors (BC). The advantages of embedded capacitors usually cited are low noises under high frequency operation and PCB's real estate saving. Zycon had also been granted a number of related patents such as U.S. Pat. Nos. 5,079,069, 5,161,086, 5,155,655, etc. However, the technology at that time could only achieve embedded capacitors with limited capacitance and therefore failed to gain widespread popularity.
To increase the capacitance of embedded capacitors, it has been suggested to first form a dielectric layer of high dielectric constant on the power or ground plane of the PCB by ion sputtering, then to form a metallic thin layer on the dielectric layer by ion sputtering or electroless plating, and finally to increase the thickness of the thin metallic layer as an electrode by electroplating. The teaching is able to achieve high capacitance from embedded capacitors by significantly reducing the distance between the electrodes and by adopting dielectric material of high dielectric constant.
However, this teaching has certain reliability issues. The resin glass fabrics, a major material for the PCB, has different thermal expansion coefficient from those of the copper foil and the dielectric material. The differences are even more significant when the temperature is beyond the glass transition temperature. Therefore, when a heat source is involved (for example, for soldering) in the PCB manufacturing process after the embedded capacitors are formed, the stress resulted from the differences of the thermal expansion coefficients would cracks the dielectric layer. Further, due to copper's Conductive Anodic Filament (CAF) effect, the copper foils at the two sides of a crack of the dielectric layer would be shorted after a period of usage. The embedded capacitor, and the entire PCB, therefore fails.